1. Field of the Invention
The present invention relates to new polymeric piperazinamides, to reverse osmosis membranes based on said polymeric piperazinamides, and to a process for the preparation of such membranes.
2. Description of the Prior Art
Membranes with selective permeability are known. Likewise, the principle of reverse osmosis is well known. According to this principle, if a solution of one or more solutes in a common solvent is pumped against a permoselective (semipermeable) membrane at a pressure greater than the osmotic pressure of the solution, one can effect separation of the components of the solution.
For a considerable time the reverse osmosis technique has been applied in desalinization of brackish water and sea water. For this purpose the membranes that are used must permit the passage of water therethrough while at the same time rejecting the salts dissolved therein.
Reverse osmosis membranes, to be practicable and economical, should be capable of permitting a high flow of water therethrough, yet should also exhibit a high capacity for rejection of dissolved salts. Membranes suitable for such use may have various different physical structures. The polymeric material that forms the membrane may, in fact, exhibit either:
(1) A DENSE AND HOMOGENEOUS STRUCTURE IN THE FORM OF A SUPPORTED ULTRA THIN FILM, OR IN THE FORM OF A HOLLOW FIBER; OR
(2) A NON-HOMOGENEOUS STRUCTURE IN THE FORM OF AN "ANISOTROPIC GEL" MEMBRANE OR A "SKIN-LIKE" MEMBRANE, CONSISTING OF A THIN SURFACE LAYER OF A SUPERFICIAL DENSE AND HOMOGENEOUS POLYMER HAVING A THICKNESS GENERALLY OF FROM 0.1-0.2.mu. OR LESS, AND A POROUS SUBSTRUCTURE, THE FUNCTION OF WHICH IS TO SUPPORT THE THIN LAYER.
It is known that the high water permeability of the "anisotropic gel" membranes (or skin-like membranes) is in part due to their low thickness, which is also responsible for the desalinating capacity of the membrane. The term "anisotropic" means that the thin and homogeneous layer, which is active in desalination, is present only on one of the two faces of the membrane.
The polymeric material usually used in the preparation of anisotropic membranes is cellulose acetate. Indeed, cellulose acetate membranes are generally used in the practice of reverse osmosis.
Besides cellulose acetate, only a few other polymeric materials are known to be capable of yielding anisotropic gel membranes (or skin-like membranes) which can give rise to a great flow of water and a high saline rejection.
In Italian Pat. Nos. 868,524 and 868,525, it is taught that polyamides derived from piperazine may be usefully employed in the preparation of membranes suited for desalinization of water by means of reverse osmosis.
While the polymers described in those patents may be transformed into non-anisotropic gel type membranes, these membranes exhibit characteristics that are not altogether satisfactory.
The present invention provides polymeric piperazinamides capable of being easily transformed into "anisotropic gel" membranes. The invention further provides a process for the preparation of anisotropic membranes for reverse osmosis based on polymeric piperazinamides.